The present invention relates to a system for controlling the flow and temperature of a fluid in an indirect type of solar collector.
Indirect solar collectors cycle a collector fluid such as antifreeze through a closed loop including the collector itself and a heat exchanger. The heat exchanger is used to heat a second fluid or element, often water which is stored in a hot water tank. An expansion tank and pressure relief valve are incorporated in the closed loop to accommodate fluid expansion and vaporization. An air vent is usually included as well to accommodate underpressure.
Indirect solar collectors are subject to overheating in several respects. During periods of low usage, the collector fluid will undergo little heat loss at the heat exchanger, causing the temperature and thus the vapor pressure of the collector fluid to rise. When the collector fluid reaches the pressure necessary to actuate the pressure relief valve, the collector fluid will be lost through the relief valve and insufficient fluid will remain to operate the system efficiently when use level increases. The collector fluid may also become corrosive when excessively heated and damage the collector and other system componenets. During periods of moderate usage, but above average sunlight, the second fluid may become overheated. When water is the second fluid, excessive heating damages the liner of the hot water storage tank, and shortens its useful life. Pump failure may cause the collector fluid in the collector itself to be overheated and damage the collector. To avoid overheating, indirect solar collectors typically have complex and expensive active cooling systems which are electronically controlled, and in large measure defeat the desired simplicity of the system.